In general, a battery pack 3 shown in FIG. 9a is removably mounted to a charging type electric tool 2 of FIG. 8. Typically, a battery pack having nickel-cadmium batteries (Ni—Cd batteries) or nickel metal hydride batteries (Ni-MH) therein is used as the battery pack 3. Here, the electric tool 2 of FIG. 8 is a pistol type impact driver, in which a handle 7 is extended from one end of a main body 6 in a direction crossing the main body 6. The electric tool 2 is constructed such that an output unit 8 is operated by drive force transmitted from a motor (not shown), which is installed in the main body 6, through a reducer (not shown). Furthermore, a battery pack mounting unit 9 is provided at the end of the handle 7 that is opposite of the main body 6. A battery pack is removably mounted to the battery pack mounting unit 9. Herein, examples of a charging type electric tool and a charging device for charging a battery pack of the charging type electric tool are disclosed in Patent Reference 1 (Japanese Patent Laid-open Publication No. 2005-342796).
In recent years, to reduce the size of the battery pack, a battery pack having lithium-ion batteries has been developed for use. Therefore, at present, electric tools using the former and the latter battery pack are available. Such battery packs are charged, for example, by the charging device 1′ of FIG. 9b. 
However, since each of the above battery packs must be charged by a dedicated charging device corresponding to each individual battery pack, a number of different charging devices are required. Therefore, when employing electric tools having different types of battery packs, it is inconvenient for users as the same number of charging devices are required.